(a) Field of the Invention
The present invention relates to an improvement in the structure of surge arresters of the single column type, be they station or distribution surge arresters.
(b) Description of the Prior Art
Surge arresters are well known electric protection devices intended to be connected in parallel across an electric apparatus to be protected, in order to reduce overvoltages that can be produced between the terminals of the latter. More specifically, surge arresters are electric systems normally existing as an open circuit which transforms itself into a closed circuit in parallel with the apparatus to be protected as soon as a significant overtension appears between the terminals of the latter. It thus makes it possible to reduce the insulation level of electrical apparatuses which they protect and, consequently, their manufacturing cost.
Surge arresters presently available on the market are widely used in networks for the transmission and distribution of electrical energy (station and distribution surge arresters). These surge arresters usually comprise a porcelain casing having the general shape of a cylindrical tube sometimes closed at one end, which casing defines a chamber in which is placed one or several columns of disk-like varistors stacked one upon the other. It is well known that varistors are electrically active elements made of metal oxide such as zinc oxide, or yet silicium carbide of which the impedance varies non-linearly under the action of an overvoltage such as to provide an adequate protection.
As any electric devices, surge arresters are sometimes subject to failure. When such a failure occurs, one or several of the varistors become short-circuited permanently and an electric arc is formed within the casing which generates explosive overpressures as well as temperatures exceeding the melting point of all known metals. In order to reduce the risk of explosion of the surge arrester casing following an internal short-circuit, pressure limiting devices have been suggested, which are intented to transfer the electric arc to the outside by means of a diaphragm and of a nozzle which guide the hot gases generated by the overpressure. Obviously, these elements must be mounted on the porcelain casing which makes its construction relatively expensive. That is in fact why such elements are mostly found in surge arresters used in high voltage transmission stations.
In U.S. Pat. No. 4,827,370 issued on May 2, 1989 to the Applicant, a new type of cylindrical casing or envelope has been proposed to replace the porcelain casings used up to now for the construction of surge arresters. This new casing is advantageously made of a synthetic insulating material capable of withstanding a high mechanical stress, such as epoxy concrete or polymer concrete. This casing is moulded over a thin tube preferably made of glass, mounted on an electrode.
The solution proposed in this U.S. patent has the great advantage of eliminating any disadvantages related to the use of porcelain for the manufacturing of the casings and hence allowing making surge arresters for distribution networks that are much less exposed to explosion risks or to breaking of the casing and this at a cost which is comparable to that of existing distribution surge arresters made of porcelain.
It has however been noted that if, as indicated above, the solution proposed in this U.S. patent makes it possible to considerably reduce the risks of explosion, it does not totally eliminate this risk in the particular case where the surge arrester is subject to a severe thermal shock.
In order to understand this risk better as well as the solution proposed in the frame work of the present invention, it is appropriate to describe more in detail the basic structure of surge arresters of the single column type and the manner in which they react in the case of internal failure.
As mentioned previously, any single-column surge arrester comprises a casing made of an insulating material, such as porcelain or, as in the case of the above-mentioned U.S. Pat. No. 4,827,370, of an epoxy or polymer concrete; such a casing defining a generally cylindrical chamber having a wall and two ends of which at least one is not closed. In the case of surge arresters of the single-column type, the single column of varistors is arranged within the chamber. These varistors have the shape of cylindrical disks having a diameter smaller than that of the casing chamber, the disks being stacked one upon the other to form the single column. Means, usually constituted by a spring pressing on one of the bases of the column, serve to hold the disks in stacked up condition within the casing. Electric contacts or electrodes are provided at the two ends of the casing chamber to allow electrically mounting the column of varistors to the terminals of and across an electrical apparatus to be protected against overvoltages.
In order to allow for a transfer of the electric arc from the inside to the outside in case of failure of the surge arrester, a diaphragm is advantageously provided across each end of the chamber of the casing which is not closed. This diaphragm is made of a sheet of aluminum or of any other material that can tear easily under the action of an overpressure generated by an electric arc that can develop within the chamber in case of internal failure of the surge arrester, to thereby allow natural evacuation of the hot gases generated by the arc at each end of the casing which is not closed and, hence reduce the risks of explosion of the latter. Usually, a nozzle is also used upstream of the diaphragm for orienting outwardly the escaping hot gases as soon as the diaphragm is perforated.
Under normal operating conditions, current coming from the outer circuit to which the surge arrester is connected flows through the latter by passing from one contact to the other via the column of varistors.
In case of the failure of one or several of the varistors, disposed in column form within the casing, an electric arc is produced inside the chamber which creates an inner overpressure causing perforation of the diaphragm. As soon as the perforation is produced, the hot gases inside the casing may escape and are directed by the nozzle toward another electrode to thereby cause a transfer of the inner arc outwardly of the casing and, hence, free the inside of the latter from overpressures and temperatures that could cause an explosion.
In order that this safety mechanism may operate adequately, it is obviously necessary that the electric arc which is naturally created during the failure may expand inside the casing chamber and that this arc may reach the diaphragm without any obstruction in order to perforate it and thereby ensure relaxation of the hot gases necessary for the outward transfer of the arc. It is therefore essential that no mechanical blocking exists along the axis of the inner chamber of the casing, in the direction of the end of the latter which is not closed.
For this purpose, the usual practice has always been to use disk-like varistors having a diameter smaller than the inner diameter of the chamber to leave a ring of air sufficient around the disks to allow the arc to develop and reach the diaphragm, being understood that any obstruction to the passage of the arc reduces the possibility of the latter to be transferred to the outside and, hence, substantially increases the risks of explosion. Now, the possibility of such an obstruction exists in permanence in the case of fragmentation of the varistor disks, for thermal or purely mechanical reasons. It has thus been noted that, in the case of failure of one of the varistors, a substantial leakage current is generated which causes an important rise in the voltage and a severe thermal shock capable of bringing an almost diagrammatical fragmentation of certain of the disks of which the fragments then move to obstruct the annular passage provided around the column.